When is the most efficient time to reincarnate?

3 SS ranks in just 2 weeks, you're really diligent! The best time to reincarnate is when stage progression noticeably slows down. Don't hesitate to reincarnate and stack up resources when you find it too difficult to defeat enemies anymore, hmm!

Rather than getting too stuck, if you feel your progress slowing down, it's better to reincarnate right away. It doesn't make a huge difference if you clear just one or two more stages, because if you cleared up to stage 30, you'll likely hit a significant slowdown around stage 32 on the next run. So, maintaining that pace seems better. Once your weapons and pets are somewhat equipped, you'll eventually need to buy characters to clear stages, so from here on, it's pretty much a diamond farming battle.

You can reincarnate when the tree can't be killed in one hit.

I leave it on and idle. When I feel like I'm hitting a wall, I push the dungeon as far as I can, then before I turn it off, I hit rebirth, finish all the rebirth level-ups, and then turn it off. That way, it pushes to the maximum level it reached while I was offline, and the attack power level-up also happens automatically to a certain extent.